The present invention relates to a system for transmitting the power of an engine to four wheels of a four-wheel drive vehicle, and more particularly to a system for controlling traction of the vehicle.
In a conventional four-wheel drive vehicle having a central differential, which is called a full time four-wheel drive vehicle, the central differential operates to uniformly split the torque of the engine for the front wheels and rear wheels.
It is known that the distribution ratio of torque influences driveability, steerability, stability, starting characteristic and others. For example, the torque is split in accordance with the distribution of load on the front and rear wheels of the vehicle for good traction. If the torque split is controlled at cornering, a stable cornering characteristic can be obtained. Accordingly, the understeer characteristic encountered in a vehicle driven by front wheels and the oversteer characteristic encountered in a vehicle driven by rear wheels can be restrained. Further, on slippery roads, the distribution of the torque to the rear wheels can be increased so as to ensure safe driving without locking the differential mechanism.
Japanese Patent Laid Open No. 55-83617 discloses such a transmitting system, in which the distribution ratio of torque for the front wheels to rear wheels is controlled by electronically controlling a central differential with a clutch or brake.
However, the system has only a central differential as a means for distributing the torque. Therefore, the distribution ratio is constant. In addition, the system can only select a two-wheel drive mode, four-wheel drive mode or a differential locking mode and is not intended for controlling the distribution ratio of torque.
The applicant of the present invention proposes various electronically controlled torque split systems where the torque distribution ratio to the front and the rear wheels can be changed.
However, although two-wheel slipping can be prevented by controlling the torque distribution ratio, slipping of all of four wheels can not be prevented by the torque split control. In order to restrain the slipping of four wheels, for example, the power of the engine is reduced to decrease the traction.